1. Field of the Invention
The present invention relates to a mobile communication system, and more particularly to an intra-frequency handoff method in a mobile communication system based on a code division multiple access (hereinafter, referred to as xe2x80x9cCDMAxe2x80x9d)
2. Discussion of Related Art
Generally, in a CDMA mobile communication system, all base stations share the available frequency (FA) and each base station has sectors or areas in which service is provided to mobile stations. The probability of call drops for a mobile station which moves between base stations having different FA is relatively high. Thus, the CDMA mobile communication allows a number of respective base stations to have common number of FA within overlapping zones of operation.
When a mobile station moves from one base station to another, a handoff allows a continual communication by switching radio lines to connect speech paths between the base stations. A handoff may largely be classified into two kinds as follows.
A soft handoff occurs when a mobile station moves from one base station to another base station wherein the base stations have a common FA. In the soft handoff, a communication with a new base station begins before communication with a base station currently in service ends, thereby maintaining the traffic path. The system may allow a use of two traffic paths by utilizing a rake reception function of the CDMA. In such case, a handoff between sectors of a same base station is known as a softer handoff.
A hard handoff occurs when a mobile station moves from one base station to a base station belonging to a different mobile switching center (MSC) wherein the base stations have traffic channels of different FA or traffic channel of different frame offsets. In the hard handoff, a communication with a new base station ends before communication with a base station currently in service starts.
Such handoffs as described above occurs frequently in a CDMA mobile communication system having a multiple frequency environment. FIG. 1 is a flow chart showing messages between a mobile station and a base station during a handoff based on a mobile assist handoff (MAHO) method of the CDMA mobile communication system in the related art. The MAHO method is initiated by a request of a handoff from a mobile station. In the MAHO method, a soft, a softer or a hard handoff is executed according to factors such as a resource relation between a base station servicing a current call and an adjacent base station, and a systematic construction of the adjacent base station. The operation of the MAHO method in a CDMA mobile communication system during a handoff is as follows.
Referring to FIG. 1, mobile station first checks the strength of the same frequency pilot signal from an adjacent base station or cell, and transmits a pilot strength measurement message (PSMM) to a base station through a traffic channel servicing the current call (S100). The base station and a base station controller analyze the transmitted PSMM and the information of the adjacent base station which the base station already possesses, to decide whether an ADD handoff procedure for establishing a new traffic path or a DROP handoff procedure for withdrawing one of an established traffic paths should be executed. Thus, an extended handoff direction message (EHDM) is transmitted to the mobile station (S101).
A pilot PN included in the transmitted EHDM is registered in an active set of the mobile station, and according to the registration, the mobile station executes for example an establishment/withdrawal of a new traffic path or an establishment/withdrawal of a simultaneous traffic path. After the execution, the mobile station transmits a handoff complete message (HCM) to the base station (S102). In the above-described MAHO method, the mobile station can transmit the PSMM only when the frequency of a current traffic call and the frequency used in a destination adjacent base station are common,
FIG. 2 is a block diagram showing a CDMA mobile communication system having an environment of a multiple frequency. Referring to FIG. 2, the CDMA mobile communication system includes a mobile station MS 207, base stations 204xcx9c206 for taking charge of a radio interface with the mobile station 207, control stations BSC 202 and 203 for controlling the base stations 204xcx9c206 and for processing a vocoding of a voice packet, a mobile switching center MSC 200 for translating a subscriber""s number to switch a call, and a home location register HLR 201 for storing and processing positional information of subscribers required for a paging.
A CDMA mobile communication system of the multiple frequency environment occurs in a mass subscribed area and non-mass subscribed neighboring area as the number of the subscribers increases. For example, suppose that a cell A under the charge of the first base station 204 is a massed-subscribed area and cells B and C respectively under the charge of the second and third base stations 205, 206 are non-massed-subscribed areas. The mobile station 207 positioned at the neighboring area of the first and second base stations 204, 205 would have the multiple frequency environment.
In the multiple frequency environment, if the number of frequencies used in the first base station 204 and the number of frequencies used in the second and third base stations 205, 206 are different, a mobile station having a call established using a non-common frequency may move from the first base station 204 to the second base station 205. In such case, an intra-frequency handoff should be performed.
However, the intra-frequency handoff cannot be performed in the MAHO method as described above, because the mobile station cannot transmit the PSMM when the frequency is different from the frequency of an adjacent base station. To solve this problem, a pilot generating equipment capable of generating a pilot may be additionally implemented in every base station positioned within the multiple frequency environment. Particularly, a pilot generating equipment is capable of transmitting a pilot in all frequencies used in a multiple frequency environment including the frequencies used by the first base station, i.e. the boundary base station, and the non-common frequencies Thus, the pilot generating equipment would generate the PSMM when a mobile station moves to the second base station, i.e. the destination base station.
As a result, the base stations 204xcx9c206 and the base station controllers 202, 203 analyze the generated PSMM to execute the intra-frequency handoff procedure when the PSMM is generated through the pilot generating equipment. The intra-frequency handoff method based on a conventional technique is as follows.
The pilot generating equipment for transmitting a pilot of frequency during conversation is already installed in the destination base station or the second base station 205. Thus, when the mobile station moves from the first base station 204 to the second base station 205, the pilot generating equipment of the second base station 205 generates pilot for a corresponding frequency. Based upon the generated pilot, the mobile station can measure a pilot signal strength of the second base station 205. Accordingly, when the pilot signal strength is larger than a comparison value from the first base station 204 and the base station controller 202, the PSMM is transmitted to the first base station 204 as a decision message to execute a handoff process.
Subsequently, the first base station 204 analyzes the PSMM and recognizes the intra-frequency handoff. If the signal strength of the second base station 205 meets the condition to perform the intra-frequency handoff, the first base station 204 establishes a new traffic path between a corresponding mobile station and the second base station 205 by using the EHDM and the HCM. Simultaneously, the first base station 204 withdraws its own traffic path.
However, the intra-frequency handoff method as described above has problems when numerous boundary base stations exist in a neighboring area of the multiple frequency environment. First, the systematic operation becomes complicated when additional pilot generating equipments are utilized to generate the pilot of a frequency during conversation. The installation of the pilot generating equipment in every the boundary base station within the multiple frequency environment not only complicates the maintenance, but increases the cost of the system.
Also, the CDMA mobile communication system may be implemented with a plurality of micro base stations (micro BTS) to manage a capacity problem of a specific area As a result, the conventional intra-frequency handoff control method would require individual installments of separate hardware apparatuses such as a pilot beacon in the micro base stations, Accordingly, additional pilot generating equipment would be difficult to apply in a handoff performed with the micro BTS. Moreover, a frequency transmitter of the pilot generating equipment installed for the intra-frequency handoff must be increased whenever the number of frequencies in the system is increases, thereby further complicating the system.
Accordingly, an object of the present invention is to solve at least the problems and disadvantages of the related art.
An object of the present invention is to provide an efficient and simple intra-frequency handoff method.
Another object of the present invention is to provide an intra-frequency handoff method without a use of a pilot generating equipment in a CDMA mobile communication system having a multiple frequency environment.
Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill, in the art upon examination of the following or may be learned from practice of the invention. The objects and advantages of the invention may be realized and attained as particularly pointed out in the appended claims.
To achieve the objects and in accordance with the purposes of the invention, as embodied and broadly described herein, an intra-frequency handoff method in a mobile communication system with a first base station providing a service for a mobile station and a second base station adjacent to the first base station, comprises (1) detecting by the first base station a handoff occurrence of a mobile station having a call established using a common frequency with the second base station accumulating strength of signals used in the detected handoff and extracting handoff career data; (2) comparing a threshold value determined from the extracted handoff career data and a signal strength value received from a mobile station having a call established using a non-common frequency with the second base station to decide a point of time for the handoff; and (3) executing a handoff for the mobile station using the non-common frequency according to the decided handoff point of time.
Preferably, the first step of the intra-frequency handoff method includes recognizing through a program load data loaded during a systematic initialization that the first base station is itself a boundary base station positioned in the multiple frequency environment; initializing the handoff career data predetermined in the first base station; detecting in the first base station whether or not a mobile station handoff to the second base station, among mobile stations having a call established using a common frequency between the first and second base stations, occurs; discriminating a type of the detected handoff when a mobile station handoff to the second base station is detected; analyzing a PSMM used in executing the detected handoff according to the discriminated type; and accumulating the signal strength extracted according to the analyzed result of the initialized handoff career data.
Also, the second step in the intra-frequency handoff method includes selecting a call anticipated for an occurrence of an intra-frequency handoff; requiring the mobile station having the selected call to transmit the PSMM; determining a given threshold value Threshpilot on the standard of the extracted handoff career data when the PSMM is received by the above requirement; comparing the given threshold value Threshpilot with a magnitude of the received PSMM; and deciding a point of time for the handoff of the selected call according to the comparison result.
Furthermore, the handoff in third step of the intra-frequency handoff method includes pre-performing handoff so that a mobile station using the non-common frequency uses a common frequency with the second base station; and executing the handoff to the second base station according to a soft handoff procedure between the first and second base stations.